This is a triomino piece:
(A 2 x 2 cell square with one of the corner cells removed)

Prove that a square, 2^n cells to the side, with one square cell removed from the corner can be covered with triomino pieces without any overlapping or going over the border for any natural value of n. The triominos can be rotated.

(For example if n = 1, the result is a triomino shape to begin with - a 2 x 2 square with one cell removed.)